Many recent external devices to which an image sensing apparatus such as a video camera, camcorder, and scanner transfers an image signal have a suspend/resume function of saving, in a storage device, pieces of information necessary for execution of an OS and application that run on the external device when power-OFF is requested, and restoring these pieces of information when the apparatus is powered on again, thereby instantaneously reproducing the state immediately before the power-OFF request. This is mainly because power consumption of the external device can be suppressed.
When an external device having the suspend/resume function is set in a suspend state, an image sensing apparatus connected to the external device stops reading an image. When the external device is resumed, image reading is enabled again.
In the above arrangement, however, when the external device is set in the suspend state, pieces of information necessary for execution of the OS and application which are running in that state are saved in a storage device and then powered off, thus the external device interrupts the read application and also stops communication. Hence, when the image sensing apparatus is performing image read operation, the image sensing apparatus cannot transfer the image to the external device, so the image read operation is wasted. When the external device is resumed, image reading is enabled again. In this case, the positional relationship between an optical unit before the external device is set in the suspend state and that after the resume is indefinite, so image reading after the resume fails.